Heat stabilizing of cellulosic insulation in electrical apparatus



United States Patent 2,722,561 STABILIZING 0F CELLULGSIC INSULA- TION IN ELECTRICAL APPARATUS HEAT This invention relates to electrical apparatus embodying cellulosic insulation and, in particular, to the stabilizing of the cellulosic insulation in contact with liquid dielectrics.

It is well known that cellulosic materials, such as paper, cotton cloth, cotton tape and wood deteriorate rapidly at temperatures of above 100 C. when in contact with liquid dielectrics such as oil. Thus, paper after being immersed for five weeks in oil at 120 C. will have only about 13% of its original tensile strength. While a strip of paper originally may be bent or flexed several hundred times before breaking, after several weeks immersion in hot oil at 100 C., it will break after twenty bends or less. If the oil in which the paper is immersed contains certain well-known stabilizers such as phenyl-alphanaphthylamine, the paper strips will be so weakened after five weeks in oil at 100 C. that they will break on the second bend.

One of the factors involved in setting the maximum temperatures to which electrical apparatus can be subjected with safety during the operation is the ability of paper and cotton tape to retain some appreciable strength after being subjected to such maximum temperatures. For this reason, electrical apparatus employing organic insulation has specified that the continuous operating temperatures do not exceed above 105 C. This limitation applies to cellulosic insulation in contact with or immersed in liquid dielectrics, though such insulation at times deteriorates at a far greater rate than does the same cellulosic insulation not in contact with oils or other liquid dielectrics.

The object of this invention is to provide for increasing the thermal stability of cellulosic insulation in contact with liquid dielectrics by applying to the dielectrics certain stabilizing compounds.

A further object of the invention is to provide for improving the physical strength of cellulosic insulation of electrical apparatus in which a liquid dielectric comes in contact with the cellulosic insulation by applying to the liquid dielectric urea or non-acidic compounds derived from urea.

Other objects of the invention will in part be obvious and will in part appear hereinafter. For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description and drawing, in which:

Figure 1 is a view in elevation, partly broken, of a transformer, and

Fig. 2 is a graph plotting strength against weeks of aging of dried manila paper in oil at 135 C.

It has been discovered that liquid dielectrics in contact with cellulosic insulation may be treated with one or more non-acidic compounds having the group in which a total of at least two hydrogen atoms are directly attached to the C and N atoms of this group, to reduce greatly the rate of loss of strength of the cellulosic insulation. Examples of the stabilizing compounds suitable for this purpose are urea, the substitution derivatives of urea and reaction products of urea. Examples '0 ice of such compounds are thiourea, hexamethylenetetramine, guanidine carbonate, tertbutyl urea, tertamyl urea, nbutyl urea, l,l-diethyl urea, 1,3-diethyl thiourea, 1,3- diisopropyl thiourea, 1,3-dimethyl urea, 1,3-dibutyl urea, dicyanidiamide, methylol urea and biuret.

By non acidic compounds is meant compounds which when placed in water in amounts of about 1% and warmed for a few hours produce aqueous solutions of a pH of 7 or higher. Thus, water with 0.2% by weight of urea has a pH of approximately l0. Dicyanidiamide under the same conditions gives a pH of 8, while thiourea gives a solution having a pH of 7. On the other hand dithiobiuret gives a pH of 5 and uramil a pH of about 1, and both of these have been found to be useless in retarding the rate of deterioration of cellulosic materials in liquid dielectrics.

In order to increase the thermal stability of cellulosic insulation in contact with liquid dielectrics, thereby greatly reducing its rate of loss of strength, a small quantity of the stabilizing compound is introduced into the liquid dielectric or otherwise placed in contact with the liquid dielectric. The quantity of the compound need be quite small. For example, a quantity equal to 0.01% by weight of the liquid dielectric is sufficient; however, a larger amount does no harm and may be desirable in some cases. One or more of the compounds may be present in an amount of up to 10% of the weight of the dielectric. The compound may be finely divided. The compound need not be appreciably soluble in the dielectric, and in fact it was found that even though urea is only very slightly soluble in cold oil, it produced excellent results. At higher temperatures as, for example, at C., urea will melt and disperse readily in oils.

Any suitable means for introducing the stabilizing compound may be employed; thus a quantity of the compound may be poured loosely into the liquid dielectric or else a porous envelope or package of the compound, such, for example, as a cloth bag filled with urea may be suspended in the liquid dielectric. In some cases, urea may be incorporated into the electrical windings by sifting it between the paper or cotton insulation as it is being wrapped so that the urea is retained mechanically between the folds of the paper or tape.

Referring to Fig. l of the drawing, there is illustrated a transformer operating in accordance with the invention. The transformer comprises a tank 10 carrying a support 12 internally on which magnetic core 14 and a coil 16 is disposed. The coil 16 comprises a high voltage Winding 18 and a low voltage winding 20 insulated from one another by cellulosic insulation 22 which comprises paper, cotton, or other cellulosic material. An exterior cellulosic wrapping 24 of cloth or paper may be applied to the coil 16. In some cases, pressboard, wood or cardboard spacers or various other cellulosic products may be applied to the electrical windings. A liquid dielectric 26 is disposed within the tank 10 to cover the core 14 and coil 16 in order to insulate them and to dissipate the heat produced in operation. The liquid dielectric may be any suitable dielectric such as oil or chlorinated diphenyl. Disposed in the casing 10 is a cloth bag 28 containing a quantity of the non-acidic compound having a H group with at least two hydrogen atoms attached directly to the atoms in the group. A cotton cloth, preferably of a loose mesh construction, is suitable. suitable material or other type of holder other than a bag may be used. For most purposes from 5 to 50 grams of urea per gallon of dielectric liquid 26 is an adequate amount.

In operation, the cellulosic insulation 22 and 24 will Any other tend to deteriorate rapidly in strength and flexibility because of the temperature to which it is subjected and because of its contact with the dielectric liquid. Thus, even after five weeks at 100 C., kraft and manila paper loses an average of approximately 52% of its strength in highly refined transformer oil. After five weeks in oil at 120 C., the tensile strength of paper will be only 13% of its original strength, and it can be flexed only a few times before breaking.

Referring to Fig. 2 of the drawing, there is illustrated the result of an accelerated aging test, wherein the curve A is a plot of the strength remaining in 5 mil. thick manila paper, such as is employed for transformer insulation, against weeks of aging at 135 C. in transformer oil. The curve A shows a rapid decrease in strength so that in twenty weeks less than of the original tensile strength remains. Moreover, the paper is so brittle that it cannot be handled without breaking and disintegrating almost at touch after this period of aging. Thus where the manila paper originally may be bent sharply back and forth several hundred times before it breaks into two pieces, after five weeks at 120 C., it will stand only a single bend.

The benefits conferred by the addition of urea to oil are illustrated in the curve B of Fig. 2 of the drawing where 10 grams of urea per gallon of the oil prevents or minimizes thermal deterioration of the same manila paper remarkably well at 135 C. Thus, after thirty weeks at this temperature over 80% of the original strength of the paper is retained. This test temperature is considerably above the normal 105 C. limit. Experience indicates that at 105 C. it would require over 5 years to reduce the strength of the paper to this 80% value. Thus after 30 weeks at 110 C. in oil treated with urea, only a 2% loss in tensile strength has been observed. The number of bends to break paper strips is over one hundred for paper aged at 135 C. for five weeks.

In disassembling transformers, after two years service, wherein the paper insulation has been in contact with oil at temperatures at no time in excess of 105 C. throughout this period, the paper has been found to be so low in strength (average 2.6 pounds per inch of width as against an original value of 26 pounds) and brittle that it breaks and falls apart on any attempt to unwrap it. The cotton tape was also very weak and could not be reused, its strength dropping from 20.6 pounds to 2.6 pounds per inch of width. Even wood spacers are badly deteriorated under these conditions. In transformers employing liquid dielectrics with additions of the stabilizing compounds, such as urea, disassembly after two years service will be possible with no failure of the paper or cotton tapes, and the wood spacers may be reused.

The following table illustrates the flexibility of paper, originally capable of withstanding over 100 bends, after being aged in oil containing 1% of the indicated compound.

Table I Temper- Bends Compound Added ature, to

0. Break Oil alone 100 Phenyl-alpha-naphthylarnine 100 5 2 Urea 100 15 100 Hexamethylenetetramine 100 15 100 Guanidine carbonate. 100 15 100 Tertbutyl urea 100 15 100 A-stage Urea Formaldehyde resin (essentially methylol urea) 100 15 100 'Ihiourea 100 15 100 n-Butyl urea 100 15 100 1,1-Diethyl urea. 100 15 100 1,3-Diethyl urea. 100 15 100 1,3-Diethyl thiourea 100 15 100 1,3-Diisopropyl thioure 100 15 100 1,3-Dimethyl thiourea. 100 15 100 1,3-Dibutyl thiourea 100 15 100 The dielectric properties of the liquid dielectrics are benefited by the presence of the stabilizing compounds disclosed herein, thus oil after being aged C. showed the following power factor values:

Table II N o Urea- 0.4% Urea- Time of Aging, Days 5:82;; ?;825; Percent Percent disclosed herein may be applied to capacitors, reactors, cables, switchgear and other electrical apparatus in which the liquid dielectric is in contact with cellulosic insulation.

It is intended that all matter contained in the above description and in the accompanying drawing shall be deemed to be illustrative and not limiting.

I claim as my invention:

1. Electrical apparatus comprising, in combination, an electrical winding developing heat during use of the apparatus, cellulosic insulation being present in the apparatus in contact with the winding and subject to loss of physical strength with passage of time at the temperatures developed, a liquid dielectric consisting essentially of petroleum oil applied to the winding and in contact with the cellulosic insulation to dissipate the heat developed therein, and urea in the liquid dielectric to maintain the physical strength of the cellulosic insulation, the urea being present in an amount of between 0.01% and 10% of the weight of the oil.

2. Electrical apparatus comprising, in combination, an electrical winding developing heat during use of the apparatus, cellulosic insulation being present in the apparatus and in contact with the winding and subject to loss of physical strength with passage of time at the temperatures developed, a liquid dielectric consisting essentially of petroleum oil applied to the winding and the cellulosic insulation to dissipate the heat developed therein, and, in the liquid dielectric to maintain the physical strength of the cellulosic insulation, a non-acidic compound having the group and a total of at least two hydrogen atoms being directly attached to any of the N and C atoms in the group, the compound being selected from the group consisting of urea, thiourea, hexamethylenetetramine, guanidine carbonate, tertbutyl urea, tertamyl urea, n-butyl urea, 1,1- diethyl urea, 1,3-diethyl thiourea, 1,3-diisopropyl thiourea, 1,3-dimethyl urea, 1,3-dibutyl urea, dicyanidiamide, methylol urea and biuret, the compound being present in an amount of from 0.01% to 10% of the weight of the liquid dielectric.

References Cited in the file of this patent UNITED STATES PATENTS 1,931,455 Clark Oct. 17, 1933 1,982,539 Reeves Nov. 27, 1934 1,994,911 Ford Mar. 19, 1935 (Other references on following page) 5 Clark Oct. 29, 1935 Missbach June 9, 1936 Missbach Feb. 2, 1937 McMahon Apr. 20, 1937 Trautman Feb. 6, 1945 5 Clark et a1. Nov. 9, 1948 Hardy Aug. 28, 1951 Hardy Aug. 28, 1951 Jenkins Aug. 28, 1951 Jackson Oct. 23, 1951 10 Hartzog Ian. 8, 1952' Hardy Feb. 3, 1953 November 1927, pages 1240-1245.

Library.)

(Copy in Scientific Vogel et al.: Electrical Engineering, vol. 61, September 1942, pages 669-673.

(Copy in Scientific Library.) 

1. ELECTRICAL APPARATUS COMPRISING IN COMBINATION, AN ELECTRICAL WINDING DEVELOPING HEAT DURING USE OF THE APPARATUS, CELLULOSIC INSULATION BEING PRESENT IN THE APPARATUS IN CONTACT WITH THE WINDING AND SUBJECT TO LOSS OF PHYSICAL STRENGTH WITH PASSAGE OF TIME AT THE TEMPERATURES DEVELOPED, A LIQUID DIELECTRIC CONSISTING ESSENTIALLY OF PETROLEUM OIL APPLIED TO THE WINDING AND IN CONTACT WITH TE CELLULOSIC INSULATION TO DISSIPATE THE HEAT DEVELOPED THEREIN, AND UREA IN THE LIQUID DIELECTRIC TO MAINTAIN THE PHYSICAL STRENGTH OF THE CELLULOSIC INSULATION, THE UREA BEING PRESENT 